CHAPTER VI INTRODUCTION TO THE SUB-CLASS EUTHERIA IL— EUTHERIA Definition, — Mammalia with teats. Mammary glands of seba ceous type. Heart with entirely membranous and complete right auric ulo- ventricular valve. Brain generally with a corpus cal- losum. Coracoid much reduced and not reaching sternum. No interclavicle. Vertebrae with epiphyses. Ribs double-headed. Viviparous, with a small ovum. In this group are included not only the Eutheria in the sense of Huxley, but also his Metatheria. Though the Metatheria, or Marsupials as we shall term them, undoubtedly form a most distinct order of mammals, perhaps even a trifle more distinct than most others, their differences from the remaining tribes are not by any means so great as those which separate Ornitlio- rhynchus and Echidna from all other mammals* In his well- known memoir upon the arrangement of the Mammalia/ Pro- fessor Huxley enumerated eleven characters as distinguishing the Metatheria either from the Prototheria or from the Eutheria. Of these only three were characters in which they approach the lower mammals. According to his showing, therefore, the preponderance of marsupial features are Eutherian. The three characters of Protothenan. type are (1) the presence of epipubes; (2) the small corpus callosum ; (3) the absence of an allantoic placenta, The last of these can be dismissed, in consequence of the recent discovery of an allantoic placenta in Perwnieles. The first character is apparently a valid distinction between the Marsupials 1 JVoc. Zool. Soc. 1880, p. 649.